mention HDF files in docs

[dcherian]

This is such a common question that we should address it in the docs.

Just saying that some hdf5 files can be opened with h5netcdf, and that the user needs to manually create xarray objects with everything else should be enough.

https://xarray.pydata.org/en/stable/io.html

[kmuehlbauer]

Has been added with #8012.